This invention relates to a free piston type of plunger assembly commonly utilized for fluid lift purposes in wells from which fluids are extracted including but not necessarily limited to oil and natural gas such as CO.sub.2, etc.
Generally, the foregoing type of fluid lift plunger assembly is provided with pads establishing wiping contact with the internal wall surface of an enclosing tube string during reciprocation of the plunger assembly. During descent of the plunger assembly, the pads are retracted by positive mechanical linkages from the tube string walls for bypass flow of fluids according to U.S. Pat. No. 3,273,504 to Lyles. Bypass flow is conducted through internal passages in the mandrel body of the plunger assembly under valve control according to U.S. Pat. Nos. 3,424,093, 4,239,458 and 4,030,818 to Moore, Yeats and Coles, respectively, so that the pads remain in wiping contact with the tube string at all times.
In the case of a plunger assembly having linkage retracted pads, sand, grains and other substances from a fractured formation within which the tube string is installed often become wedged and trapped between moving parts of the retracting linkage mechanism to adversely affect performance. Further, the provision of such retracting mechanism makes manufacture of the plunger assembly costly while repair, parts replacement and maintenance sometimes becomes economically prohibitive. In the case of plunger assemblies through which internal bypass flow passages are established by valve control, other problems arise because of pad contact during descent, including tube string wear, deterioration of pad sealing and valve malfunction.
It is therefore an important object of the present invention to provide a plunger assembly that is inherently less costly to fabricate, repair and maintain despite the reliance on pads retracted during descent in order to establish a bypass flow passage, thereby avoiding the problems associated with valve controlled internal bypass flow types of plunger assemblies.
An aditional object in accordance with the foregoing object is to provide a fluid lift type plunger assembly utilizing pads that are retracted during descent without use of any complex linkage mechanism.